The present invention relates to a small type or miniature acoustic device having a water proof characteristic and a sound alarm function useable as a sounding member for an alarm wrist watch.
In alarm wrist watches and pocket alarms, there are many parts, i.e. a battery, a quart vibrator and an integrated circuit in which a high reliability is required. Particularly, as to a wrist watch exposed to severe conditions as is usual with such devices, it is necessary and preferable to provide a water proof characteristic for the watch case to maintain the reliability of the very important parts and to shut it off from outside air, therefore, a water proofed watch case is generally employed.
In the case of an alarm wrist watch, there is the problem of the watch case providing a water proof characteristic for said watch case, i.e. said watch case has a hermetic characteristic whereby a pressure difference between the outer barometric pressure and the pressure in said watch case is caused by a temperature irregularity.
FIG. 1 shows a conventional embodiment in which a sound member is mounted to a watch case, and wherein an electro-magnetic buzzer is employed as the sound member. In FIG. 1, numeral 1 is a magnetic core which is composed of a soft magnetic material, numeral 2 is a plate which is composed of the same material of that as said core 1, numeral 3 is a coil which is wound around said core 1, numeral 4 is a magnet, numeral 5 is a vibrating plate, numeral 6 is an upper yoke which is composed of a soft magnetic material, and said upper yoke is connected to said vibrating plate 5 by welding means.
Numeral 7 is a supporting member of the vibrating plate 5, numeral 8 is a watch case, numeral 9 is a packing member for water proofing.
As to a water proof construction relating to a sounding member of a watch case, as shown in FIG. 1, the packing member 9 is mounted between the vibrating plate 5 and watch case 8, said packing member 9 is pressed by a screw bolt 7a which is located in a peripheral portion of the supporting member 7 and a screw bolt 8a which is located in a watch case 8. The upper yoke 6 is, in the case of a non water-proof construction, generally located in the opposite portion of the magnetic core 1 against the vibrating plate 5. However, the yoke 6 has a problem for corrosion resistance since it is comprised of annealed magnetic material and therefore the yoke 6 is located so as to not directly contact outer air, further the vibrating plate 5 is generally composed of a material having a high corrosion resistance such as a stainless steel.
The above noted construction of a buzzer is well known and a detailed description of said problem in the above noted construction of the sounding member will now be given.
FIG. 2 shows a change of resonance frequency of the electromagnetic buzzer in FIG. 1 according to temperature variations, a dotted line "A" shows the characteristic in the case of a non-hermetic condition of a watch case, and the solid line "B" shows the characteristic in the case of a hermetic condition of a watch case. In the case of the dotted line "A" in FIG. 2, the resonance frequency curve has a little inclination in the negative direction, however, the above noted condition is generally caused by several factors including Young's modulus, and the magnetic characteristic and temperature characteristic of the magnet. On the contrary, in the case of the line "B" in FIG. 2, there is a large change of the resonance frequency, and such a large change presents a serious problem in the case of a hermitcally sealed watch case. This is caused by a pressure difference between inner pressure and outer barometric pressure, since the pressure in a watch case is changed by temperature according to an expression "P/T=constant" and the pressure is directly applied to the vibrating plate 5. In the above noted expression, "p" is pressure and "T" is absolute temperature. In a watch, such a pressure difference is about 0.23 barometric pressure in the range -10.degree. C.-+60.degree. C., i.e., the guaranteed range of operational temperature of a normal wrist watch. It will be understood that the resonance frequency is greatly changed by a small change of the gap between the core 1 and the upper yoke 6 and by a change of the vibrating plate 5 due to the pressure difference since the vibrating plate 5 is very thin, such as 0.05 cm. Further the above noted problem is similarly caused in a piezo electric type buzzer.
In a small type acoustic device, the buzzer and timepiece functions are operated by a small battery, thereby it is necessary to keep a high electro-acoustic transducing efficiency of the buzzer as an alarm function and for this purpose the resonance of the buzzer is generally employed.
However, with a change of the resonance frequency due to changes in temperature, a sufficient sound pressure is not produced when there is both a change of the resonance frequency of the buzzer and a change of the outer barometric pressure, and therefore one is not able to obtain an alarm function.